The present invention relates to a magneto-optical recording medium, such as a magneto-optical disk, a magneto-optical tape and a magneto-optical card, its producing method, its reading method and reading apparatus, and more particularly to a magneto-optical recording medium capable of reading out at magnetically-induced super-resolution.
A magneto-optical disk is known as a high density recording medium, and uses an external magnetic field and a laser beam to form recorded signals (bits) of submicron size on the medium. Accordingly, as compared with the conventional external recording medium such as a floppy disk and a hard disk, the recording capacity can be drastically increased. Recently, in the rapid spread of multimedia, there is an increasing demand for a magneto-optical disk of a larger capacity with the increase of the quantity of information.
To increase the recording capacity of a magneto-optical disk, that is, to enhance the recording density, it is required to reduce the bit size and narrow the bit interval in the peripheral direction. However, recording and readout of bits in the magneto-optical disk are limited by the spot diameter of the emitted light beam. To read out a small bit having a period equal to or smaller than the spot diameter, the beam spot must be reduced to a smaller size, but the beam spot is limited by the wavelength .lambda. of the light source and the number of apertures (NA) of the objective lens, and hence there is a limit to the reduction in the size.
To read out small bits having a period equal to or smaller than the spot diameter by reading out bits from a specific temperature region in a spot by making use of temperature distribution in the spot of a medium, the magnetically-induced super-resolution (MSR) readout method has been proposed in Japanese Patent Application Laid-Open Nos. 1-143041 (1989), 3-93058 (1991), 4-271039 (1992), etc.
The MSR medium capable of reading out at MSR proposed in Japanese Patent Application Laid-Open No. 1-143041 (1989) is capable of detecting bits equal to or smaller than the spot diameter by applying a magnetic field of some hundreds of oersteds (Oe) at the time of readout, but it is not advantageous for heightening the density in the radial direction, since the readout region is wide, and therefore the track pitch cannot be narrowed. The MSR medium proposed in Japanese Patent Application Laid-Open No. 3-93058 (1991) is narrower in the readout region than the MSR medium proposed in Japanese Patent Application Laid-Open No. 1-143041, but as the readout power of the light beam is intensified, the region becomes wider, and it is also required to apply an initializing magnetic field of 3.5 to 4.0 kOe. To apply a magnetic field of several kilo-oersteds, it requires a permanent magnet of material of a large energy product such as SmCo and NdFeB, and it is not only expensive, but also made difficult to reduce the size of the readout apparatus.
FIG. 1 is a diagram showing the direction of magnetization at the time of readout of a magneto-optical disk which is an MSR medium proposed in Japanese Patent Application Laid-Open No. 4-271039 (1992). This MSR readout method is known as RAD (rear aperture detection) double mask system for reading out recorded bits from an intermediate-temperature region, using the low-temperature region and high-temperature region in the laser spot S as mask regions. A recording disk 50 is composed by laminating a readout layer 51, a readout assisting layer 52, an intermediate layer 53, and a recording layer 54 sequentially on a substrate not shown.
Immediately before emitting a reading laser beam, an initializing magnetic field is applied to the magneto-optical disk by an initializing magnet 55 so that the magnetization direction of the readout layer 51 and the readout assisting layer 52 alone may be aligned with that of the initializing magnetic field. At this time, bits equal to or smaller than the laser spot diameter are recorded in the recording layer 54. When reading, in the region immediately after application of initializing magnetic field (low-temperature region), the readout layer 51 functions as mask in a state of covering the bits recorded in the recording layer 54. In the region exceeding the Curie temperature of the readout assisting layer 52 (high-temperature region) by application of laser beam, the exchange coupled force of the recording layer 54 and readout layer 51 is cut off, and the magnetization direction of the readout layer 51 of that region is aligned with the direction of readout magnetic field Hr applied from outside. As a result, in the high-temperature region, the readout layer 51 becomes a mask for covering the bits. The region enclosed by such low-temperature region and high-temperature region serving as mask (intermediate-temperature region) becomes a transfer region, and bits are read out from this transfer region.
In such conventional MSR readout method, reading from a narrow readout region is possible, and the resolution is high and track pitch is narrow, but in addition to application of a reading magnetic field of some hundreds of oersteds in the laser beam applying region, it is necessary to install an initializing magnet for generating a magnetic field of several kilo-oersteds in order to align the magnetization of both readout layer 51 and readout assisting layer 52 in the direction of initializing magnetic field. As mentioned above, the initializing magnet for generating a magnetic field of several kilo-oersteds is expensive and a large space is needed for compensating for magnetic field leakage, and the reading apparatus is increased in size.
To solve these problems, the present applicant proposed, in Japanese Patent Application Laid-Open No. 7-244877 (1995), an MSR medium composed of three magnetic layers, a readout layer, an intermediate layer, and a recording layer, laminated on a substrate. In this proposed magneto-optical disk, by specifying the material composition, film thickness and magnetic characteristic of each layer, MSR readout is enabled by application of an external field of 1 kOe or less. FIG. 2 is a graph showing temperature characteristics of exchange coupled force between magnetic layers of this magneto-optical disk. The ordinate indicates the exchange coupled force, and the abscissa denotes the temperature.
The exchange coupled force between the readout layer and the intermediate layer (hereinafter called first exchange coupled force) decreases with the increasing temperature, whereas the exchange coupled force between the recording layer and the intermediate layer (hereinafter called second exchange coupled force) increases with the increasing temperature. Accordingly, when reading, in the high-temperature region (about 180.degree. C. or more) and the low-temperature region (about 100.degree. C. or less), a common external magnetic field nearly exceeding the first and second exchange coupled forces is applied, and in the low-temperature region of the magneto-optical disk, the second exchange coupled force is cut off, and the magnetization direction of the intermediate layer is aligned with the direction of the external magnetic field to form a mask. In the high-temperature region, on the other hand, the first exchange coupled force is cut off, and the magnetization direction of the readout layer is aligned with the direction of the external magnetic field to form a mask.
In this reading method of a magneto-optical disk, reading at high resolution is enabled, and the external magnetic field is enough at some hundreds of oersteds. Hence, without having to apply an initializing magnetic field, MSR readout is realized only by application of a reading magnetic field, so that the reading apparatus may be reduced in size.
To read out such MSR medium, it is necessary to apply an external magnetic field of some hundreds of oersteds at all times. If such an external magnetic field is applied by using an electromagnet, the power consumption for reading occupies a larger portion of the power consumption of the recording and reading apparatus. Or when the external magnetic field is large, a large-sized electromagnet is needed, and the power consumption increases.
To reduce the size of electromagnet and lower the power consumption by solving these problems, MSR readout must be realized by a low reading magnetic field. For this purpose, it may be considered to decrease the second exchange coupled force of the magneto-optical disk. FIG. 3 is a graph showing the relation of the shift amount and the minimum reading magnetic field of the magneto-optical disk having the magnetic characteristics shown in FIG. 2, and the ordinate indicates the minimum reading magnetic field, and the abscissa denote the shift amount. Herein, the shift amount is a value expressing the magnetic characteristic of the magnet-optical disk, and it is a guideline value for the magnitude of the exchange coupled force. FIG. 4 is a graph showing the relation of the shift amount and the SN ratio, in which the ordinate indicates the SN ratio and the abscissa denotes the shift amount. As shown in FIG. 3, as the shift amount decreases, the minimum reading magnetic field becomes lower, and it is apparent that the mask can be formed by a lower magnetic field as the exchange coupled force is weaker. However, as shown in FIG. 4, when the shift amount is small, the SN ratio is low, and when the exchange coupled force between the recording layer and the intermediate layer is too weak, it is difficult to read the recorded bit accurately.